In the field of optical devices, anti-reflective thin film coatings are well known for reducing or eliminating surface reflections from components such as lenses and filters, for example. Anti-reflective (AR) coatings are generally designed for efficient transmission of light through the coating and the associated optical component. For improved performance, anti-reflective films have been developed with a quintic refractive index profile, as described in U.S. Pat. No. 4,583,822 issued to Southwell, the teachings of which are incorporated herein by reference. Quintic anti-reflective films are fabricated with a refractive index profile so that the initial refractive index of the film is as close as possible to the refractive index of the incident medium and the final refractive index of the film is substantially the same as that of the substrate.
In contrast to lenses and filters, reflective surfaces such as mirrors are designed to reflect incident radiation. Spectrally selective coatings may be applied to substrates to form surfaces that reflect light only in specific, predetermined, narrow wavelength bands. In such spectrally selective reflective devices, however, a means is required for absorbing light not reflected (i.e., light that is transmitted) by the coating. In the prior art, standard AR coatings have been applied to the surface, but they are effective only for narrow wavelength bands. Multi-layer light absorbing films have also been used on chromium, for example, where an absorbing but not transparent coating is needed. Such layered coatings usually provide good absorption only over a rather narrow wavelength range. Typically, these coatings absorb light in the visible region where the human eye is sensitive. At longer wavelengths (i.e., mid- and long-wave infrared) these coatings become efficient reflective surfaces rather than absorbers. Therefore, what is needed is a thin film material that absorbs light effectively over a broad range of wavelengths without causing unwanted reflections.